New research highlights tight chemical balance needed to preserve key nutrients
The favorite fable of astronomers has just taken a new twist. The “Goldilocks zone” – the region of space neither too close nor too far from a star where liquid water could exist on the surface of a planet – now has a chemical equivalent. Researchers have found that a narrow range of planetary conditions is necessary to ensure the availability of bioessential nutrients like phosphorus and nitrogen.
The team simulated tens of thousands of exoplanets and found that fewer than 1 in 10 people had Earth-like abundances of phosphorus and nitrogen.. The results could help explain why life has not yet been found beyond our home planet, planetary scientist Craig Walton and colleagues report February 9 in Natural astronomy.
Water is important for planetary habitability, but that’s not allsays Walton, of the University of Cambridge. “You need nutrients.” In particular, elements such as phosphorus and nitrogen are essential for assembling cell walls, encoding genetic information, and building proteins, among other roles. Imagining life without these nutrients is far-fetched, Walton says. “It’s really hard to imagine what alternative biology would look like.”
But even a water planet endowed with phosphorus and nitrogen of its environment of birth does not receive the scientific green light to welcome life. This is because these elements can sink into the heart of a forming planet. And unlike a planet’s mantle, which regularly exchanges material with the surface via volcanism, the core is isolated. Any phosphorus or nitrogen that seeps in is of no use to life living on the surface, says Sebastiaan Krijt, an astrophysicist at the University of Exeter in England, who was not involved in the research. “It’s completely inaccessible to life.”
Whether or not phosphorus and nitrogen enter the core depends on the availability of reactive oxygen in the mantle. “Oxygen is really what’s key,” says Laura Rogers, an astronomer at NOIRLab in Tucson, Arizona. The abundance of oxygen determines how phosphorus and nitrogen react with iron, which tends to sink deeper and deeper into a forming planet over time. When there is a lot of oxygen, phosphorus does not bind to iron and therefore tends to stay in the mantle; the nitrogen, in turn, will bind to the iron and flow into the nucleus. Low oxygen levels cause the opposite phenomenon: less phosphorus in the mantle and more nitrogen.
It’s a push-pull situation, Walton said. “You win one, you lose another.”
Walton, Rogers and their team hypothesized that there must be a “Goldilocks chemical zone” – a sweet spot of oxygen abundance that results in Earth-like amounts of phosphorus and nitrogen in a planet’s mantle. To study this idea, they simulated exoplanets with initial amounts of phosphorus and nitrogen based on the observed chemistry of several thousand nearby stars and a range of reactive oxygen levels taken from previous theoretical work.
Fewer than 10 percent of these planets had sufficient amounts of phosphorus and nitrogen in their mantles to support life, the team found. “It looks like there will be many planets that are starved of nitrogen or phosphorus,” says Walton. Reactive oxygen at levels comparable to, or even slightly above, Earth’s ended up providing the ideal conditions for maintaining the levels of phosphorus and nitrogen essential for life in a planet’s mantle, the team found.
Exoplanets are discovered all the time; more than 6,000 have been confirmed to date. But many planetary parameters must align for life to potentially take hold: in addition to the need for liquid water, the availability of oxygen must also be perfect. “This forces us to reconsider the prevalence of Earth-like planets in the cosmos,” says Krijt.
Physicist Enrico Fermi asked where all extraterrestrial life is. Perhaps the Fermi Paradox – the conundrum that the universe is vast and yet life has not been found beyond Earth – makes a little more sense now.
